Method for fabrication of semipolar (Al, In, Ga, B)N based light emitting diodes

ABSTRACT

A yellow Light Emitting Diode (LED) with a peak emission wavelength in the range 560-580 nm is disclosed. The LED is grown on one or more III-nitride-based semipolar planes and an active layer of the LED is composed of indium (In) containing single or multi-quantum well structures. The LED quantum wells have a thickness in the range 2-7 nm. A multi-color LED or white LED comprised of at least one semipolar yellow LED is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. Section 120 of commonly-assigned U.S. application Ser. No.12/419,119, filed on Apr. 6, 2009 now U.S. Pat. No. 8,148,713, byHitoshi Sato, Hirohiko Hirasawa, Roy B. Chung, Steven P. DenBaars, JamesS. Speck and Shuji Nakamura, entitled “METHOD FOR FABRICATION OFSEMIPOLAR (Al,In,Ga,B)N BASED LIGHT EMITTING DIODES,” which applicationclaims the benefit under 35 U.S.C. Section 119(e) of commonly-assignedU.S. Provisional Patent Application Ser. No. 61/042,644, filed on Apr.4, 2008, by Hitoshi Sato, Hirohiko Hirasawa, Roy B. Chung, Steven P.DenBaars, James S. Speck and Shuji Nakamura, entitled “METHOD FORFABRICATION OF SEMIPOLAR (Al,In,Ga,B)N BASED LIGHT EMITTING DIODES,”

which applications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned applications:

U.S. patent application Ser. No. 12/419,128, filed on same dateherewith, by Hitoshi Sato, Roy B. Chung, Feng Wu, James S. Speck, StevenP. DenBaars and Shuji Nakamura, entitled “MOCVD GROWTH TECHNIQUE FORPLANAR SEMIPOLAR (Al, In, Ga, B)N BASED LIGHT EMITTING DIODES,” whichapplication claims the benefit under 35 U.S.C. Section 119(e) of U.S.Provisional Application Ser. No. 61/042,639, filed on Apr. 4, 2008, byHitoshi Sato, Roy B. Chung, Feng Wu, James S. Speck, Steven P. DenBaarsand Shuji Nakamura, entitled “MOCVD GROWTH TECHNIQUE FOR PLANARSEMIPOLAR (Al, In, Ga, B)N BASED LIGHT EMITTING DIODES,” and

U.S. patent application Ser. No. 11/840,057, filed on Aug. 16, 2007, byMichael Iza, Hitoshi Sato, Steven P. Denbaars and Shuji Nakamura,entitled “METHOD FOR DEPOSITION OF MAGNESIUM DOPED (Al, In, Ga, B)NLAYERS,” now U.S. Pat. No. 7,709,284, issued May 4, 2010, whichapplication claims the benefit under 35 U.S.C. Section 119(e) of U.S.Provisional Application Ser. No. 60/822,600, filed on Aug. 16, 2006, byMichael Iza, Hitoshi Sato, Steven P. Denbaars and Shuji Nakamura,entitled “METHOD FOR DEPOSITION OF MAGNESIUM DOPED (Al, In, Ga, B)NLAYERS,”

which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to yellow light emitting diodes (LEDs) andmethods of fabricating the same.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

Current nitride technology for electronic and optoelectronic devicesemploys nitride films grown along the polar c-direction. However,conventional c-plane quantum well (QW) structures in III-nitride basedoptoelectronic and electronic devices suffer from the undesirablequantum-confined Stark effect (QCSE), due to the existence of strongpiezoelectric and spontaneous polarizations. The strong built-inelectric fields along the c-direction cause spatial separation ofelectrons and holes that in turn give rise to restricted carrierrecombination efficiency, reduced oscillator strength, and red-shiftedemission.

One approach to eliminating the spontaneous and piezoelectricpolarization effects in GaN optoelectronic devices is to grow thedevices on nonpolar planes of the crystal. Such planes contain equalnumbers of Ga and N atoms and are charge-neutral. Furthermore,subsequent nonpolar layers are crystallographically equivalent to oneanother so the crystal will not be polarized along the growth direction.Two such families of symmetry-equivalent nonpolar planes in GaN are the{11-20} family, known collectively as a-planes, and the {1-100} family,known collectively as m-planes. Unfortunately, despite advances made byresearchers at the University of California at Santa Barbara (UCSB),growth of nonpolar nitrides remains challenging and has not yet beenwidely adopted in the III-nitride industry.

Another approach to reducing, or possibly eliminating, the polarizationeffects in GaN optoelectronic devices, is to grow the devices onsemipolar planes of the crystal. The term semipolar planes can be usedto refer to a wide variety of planes that possess two nonzero h, i, or kMiller indices, and a nonzero/Miller index. Some commonly observedexamples of semipolar planes in c-plane GaN heteroepitaxy include the{11-22}, {10-11}, and {10-13} planes, which are found in the facets ofpits. These planes also happen to be the same planes that the authorshave grown in the form of planar films. Other examples of semipolarplanes in the wurtzite crystal structure include, but are not limitedto, {10-12}, {20-21}, and {10-14} planes. The nitride crystal'spolarization vector lies neither within such planes or normal to suchplanes, but rather lies at some angle inclined relative to the plane'ssurface normal. For example, the {10-11} and {10-13} planes are at62.98° and 32.06° to the c-plane, respectively.

In addition to spontaneous polarization, the second form of polarizationpresent in nitrides is piezoelectric polarization. This occurs when thematerial experiences a compressive or tensile strain, as can occur when(Al, In, Ga, B)N layers of dissimilar composition (and thereforedifferent lattice constants) are grown in a nitride heterostructure. Forexample, a thin AlGaN layer on a GaN template will have in-plane tensilestrain, and a thin InGaN layer on a GaN template will have in-planecompressive strain, both due to lattice mismatching to the GaN.Therefore, for an InGaN QW on GaN, the piezoelectric polarization willpoint in the opposite direction to the spontaneous polarization of theInGaN and GaN. For an AlGaN layer latticed matched to GaN, thepiezoelectric polarization will point in the same direction as thespontaneous polarization of the AlGaN and GaN.

The advantage of using semipolar planes over c-plane nitrides is thatthe total polarization will be reduced. There may even be zeropolarization for specific alloy compositions on specific planes. Suchscenarios will be discussed in detail in future scientific papers. Theimportant point is that the polarization will be reduced as compared tothe polarization of c-plane nitride structures. A reduced polarizationfield allows growth of a thicker QW. Hence, higher Indium (In)composition and thus longer wavelength emission, can be achieved. Manyefforts have been made in order to fabricate semipolar/nonpolar basednitride LEDs in longer wavelength emission regimes [1-6].

This disclosure describes an invention allowing for fabrication of blue,green, and yellow LEDs on semipolar (Al, In, Ga, B)N semiconductorcrystals. Although longer wavelength emission from LEDs has beenreported from AlInGaP material systems, there have been no successfuldevelopments of yellow LEDs emitting in the range of 560 nm-570 nmwavelength in both nitrides and phosphides.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesat least one yellow, amber or red LED, grown on at least oneIII-nitride-based semipolar plane, with a peak emission wavelengthlonger than 560 nanometers (nm) and an output power of more than 3.5milliwatts (mW) at a current of 20 milliamps (mA).

The LED typically comprises an active layer for emitting light, whereinthe semipolar plane enables a thickness of the active layer that issufficiently thick, an Indium (In) composition of the active layer thatis sufficiently high, and a crystal quality of the active layer that issufficiently high, so that the light has the peak emission wavelengthlonger than 560 nm and the output power of more than 3.5 mW at a currentof 20 mA. The active layer typically comprises In containing singlequantum well (SQW) or multi-quantum well (MQW) structures comprising atleast one QW, and the QW has the crystal quality sufficiently high toobtain the output power of more than 3.5 mW at the current of 20 mA. Inone example, the QW has the thickness in a range from 2 nm to 7 nmand/or the crystal quality is a Threading Dislocation (TD) density of9×10⁹ cm⁻² or less.

The present invention further discloses a multi-color LED device, foremitting multi-colored light, comprised of a yellow, amber, or red lightemitting LED (e.g., semipolar yellow LED grown on a semipolar plane),and a white LED device, for emitting white light, comprised of a yellow,amber, or red light emitting LED (e.g., semipolar yellow LED grown on asemipolar plane). The semipolar yellow LED may be a III-nitride basedLED grown on a {11-22} semipolar plane, for example.

The present invention further discloses a white LED device comprised ofa blue light emitting QW and a yellow light emitting QW grown on one ormore III-nitride based semipolar planes. Typically, the white LED devicecomprises MQWs that are comprised of the blue-light emitting QW that isa first InGaN QW, with a first In composition for emitting blue light,and the yellow-light emitting QW that is a second InGaN QW, with asecond In composition for emitting yellow light.

The present invention further discloses a method of fabricating theyellow, amber, or red LED, by growth on at least one III-nitride basedsemipolar plane, so that the LED emits light with a peak emissionwavelength longer than 560 nm and an output power of more than 3.5 mW ata current of 20 mA. The present invention also discloses a method offabricating a white LED device emitting white light, comprising growingblue-light emitting quantum wells and yellow-light emitting quantumwells on one or more III-nitride based semipolar planes. In addition,the present invention discloses a method of emitting white light from anLED, comprising emitting blue light and from blue-light emitting quantumwells and yellow light from yellow-light emitting quantum wells, whereinthe blue light emitting quantum wells and the yellow light emittingquantum wells are grown on one or more III-nitride based semipolarplanes.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a flowchart of a growth process for semipolar GaN thin filmsaccording to the present invention.

FIG. 2 is a temperature profile (temperature vs. time) for thedeposition of nitride based diode device layers containing InGaN MQWs,or a SQW, according to an embodiment of the present invention.

FIG. 3 is a graph of external quantum efficiency (EQE) vs. peak emissionwavelength (nm), and current development status, for different materialssystems using LED technology (c-plane UCSB, Nonpolar-UCSB, c-planeIn_(x)Ga_(1-x)N (wherein 0≦x≦1), Semipolar-UCSB,(Al_(x)Ga_(1-x))_(0.52)In_(0.48)P, and International Workshop on NitrideSemiconductor (IWN) 2006 (IWN06) by Nichia LEDs, wherein the wavelengthresponse of the human eye (eye sensitivity profile) is also shown.

FIG. 4( a) is a graph plotting the dependence of output power (arbitraryunits, a.u.) and Electroluminescence (EL) peak emission wavelength (nm),of EL from semipolar LEDs, on the number of QWs, or MQW periods, in theLED (for a SQW and MQW).

FIG. 4( b) is a graph plotting the dependence of output power (a.u.) andpeak EL wavelength (nm), of EL from semipolar LEDs, on the thickness ofthe QWs (nm).

FIG. 5( a) plots EL peak emission wavelength (m, full circles) and ELfull width at half maximum (FWHM) (m, open triangles) of a yellow LEDgrown on (11-22) GaN as a function of direct current (DC) currents from1 mA to 100 mA.

FIG. 5( b) plots output power (mW) and EQE (%) as a function of drivecurrent (mA) for a yellow light emitting LED containing InGaN QWs(filled diamonds and circles) and on a semipolar plane, and an AlInGaPLED (hollow triangles), for pulsed currents at 10% duty cycle and at 10KHz.

FIG. 6 is a cross-sectional schematic of a semipolar yellow, amber orred III-nitride based LED grown on a semipolar plane.

FIG. 7 is a Commission Internationale d'Eclairage (CIE) 1931 (x,y)chromaticity diagram, wherein the area within the triangle shows thepossible colors that can be generated by combining UCSB semipolar blue,green and yellow LEDs, noting that actual coordinates of each LED maynot correspond to the coordinates in FIG. 7, wherein the scale runningalong the perimeter of the triangle indicates the wavelength of light innanometers (nm), corresponding to the color co-ordinates x,y.

FIG. 8 is a schematic band structure for a white LED that can be madeusing QWs with two different bandgaps in a single structure, plottingenergy as a function of position through the layers of the white LED.

FIG. 9 is a cross-sectional schematic of a white LED comprised of one ormore blue light emitting QWs and yellow light emitting QWs grown on oneor more III-nitride based semipolar planes.

FIG. 10 is a transmission electron micrograph (TEM) image of a (11-22)yellow LED device in a [1-100] cross section, wherein the scale shows adistance of 40 nm.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The present invention describes a method for fabrication of blue, green,yellow, white, and other color LEDs with a bulk semipolar GaN substratesuch as {10-1-1}, {11-22}, and other planes. Semipolar (Al, In, Ga, B)Nsemiconductor crystals allow the fabrication of a multilayer structurewith zero, or reduced, internal electric fields resulting from internalpolarization discontinuities within the structure, as described inprevious disclosures. This invention describes the first yellow LEDs inIII-nitride based optoelectronics. Further, the invention incorporates alow temperature growth of n-type GaN resulting in improvement of filmquality. The use of a semipolar (Al, In, Ga, B)N semiconductororientation results in reduced internal electric field, and thus athicker QW and higher In composition for longer wavelength emissionsrelative to [0001] nitride semiconductors. The invention of yellow LEDsallows the fabrication of high power white LEDs.

Technical Description

Process Steps

The present invention describes a method for growth of semipolar{10-1-1} and {11-22} GaN via metalorganic vapor deposition (MOCVD), andfabrication of yellow LEDs. FIG. 1 is a flowchart that illustrates thesteps of a MOCVD growth process for semipolar GaN thin films on a{10-1-1} or {11-22} bulk GaN substrate, according to an embodiment ofthe present invention that is described in the following paragraphs.

Block 100 represents loading a substrate into a reactor. For example,for the growth of a semipolar LED structure, a bulk {10-1-1} or {11-22}GaN substrate, is loaded into a MOCVD reactor.

Block 102 represents heating the substrate under hydrogen and/ornitrogen and/or ammonia ambient. The reactor's heater is turned on andramped to a set point temperature, to heat the substrate under hydrogenand/or nitrogen ambient. Generally, nitrogen and/or hydrogen flow overthe substrate at atmospheric pressure.

Block 104 represents depositing an n-type nitride semiconductorfilm/layer (in this case n-type GaN) on the substrate. After the heatingstep of block 102, the temperature is set to 1100° C., and 54mmol/minute (micro-mole per minute) of trimethyl-gallium (TMGa) isintroduced into the reactor with DiSilane for 30 minutes to initiate thegrowth of n-type GaN. 4 slm (standard liters per minute) of ammonia(NH₃) is also introduced at this stage and it is kept at the same leveluntil the end of the growth.

Block 106 represents depositing a nitride MQW on the n-type nitridelayer. Once the desired n-type GaN thickness of block 104 is achieved,the reactor's temperature set point is decreased to 815° C., and 6.9mmol/minute of Triethylgallium (TEGa) is introduced into the reactor anda 20 nm thick GaN barrier layer is grown. Once the desired thickness ofGaN barrier is achieved, 10.9 mmol/minute trimethyl-indium (TMIn) isintroduced into the reactor to deposit a 3 nm thick InGaN SQW. After thedeposition of the InGaN layer, 6.9 mmol/minute of TEGa is againintroduced into the reactor for growth of GaN to finalize the QWstructure. This step can be repeated several times to form a MQW.

Block 108 represents depositing an electron blocking layer. Once theSQW/MQW is deposited, 3.6 mmol/minute of TMGa, 0.7 mmol/minute oftrimethyl-aluminum (TMAl), and 2.36×10⁻² mmol/minute of Cp₂Mg areintroduced into the reactor in order to form a 10 nm-thick AlGaNelectron blocking layer which is slightly doped with Mg.

Block 110 represents depositing a low temperature nitride p-typesemiconductor film on the SQW/MQW. Once a desired AlGaN thickness isachieved in block 108, the reactor's set point temperature is maintainedat 820° C. for 10 minutes. For the first 3 minutes of this interval,12.6 mmol/minute of TMGa and 9.8×10⁻² mmol/minute of Cp₂Mg areintroduced into the reactor. For the last 7 minutes, the flow of Cp₂Mgis doubled. Then, the temperature is ramped to 875° C. in 1 minute, andTMGa flow is kept constant and Cp₂Mg is reduced back to 9.8×10⁻²mmol/minute during this ramp time. The growth of p-GaN is continued at875° C. for another 1 minute.

Block 112 represents annealing the film (specifically, the p-GaN) in ahydrogen deficient ambient gas. Once the reactor has cooled, the nitridediode structure formed in blocks 100-110 is removed and annealed in ahydrogen deficient ambient for 15 minutes at a temperature of 700° C. inorder to activate the Mg doped GaN (p-GaN).

Block 114 represents the end result of the method, an (Al,Ga,In,B)Ndiode film with longer wavelength emission, wherein the layers grown inblocks 104-110 are typically grown in a semipolar orientation, forexample by growing on a semipolar plane of the substrate of block 100.For example, this method can be used to fabricate a yellow, amber, orred LED, comprising growing the LED on at least one III-nitride-basedsemipolar plane, so that the LED emits light with a peak emissionwavelength longer than 560 nm and an output power of more than 3.5 mW ata current of 20 mA. Typically, the semipolar plane enables a thicknessof the active layer that is sufficiently thick, an Indium (In)composition of the active layer that is sufficiently high, and a crystalquality of the active layer that is sufficiently high, so that the lighthas the peak emission wavelength longer than 560 nm and the output powerof more than 3.5 mW at a current of 20 mA. The crystal quality may be adislocation density (e.g., TD) of 9×10⁹ cm⁻² or less, for example. Thequantum well may have the thickness in a range from 2 nm to 7 nm, forexample.

This method can also be used to fabricate a white LED device emittingwhite light, comprising growing blue-light and yellow-light emitting QWson one or more III-nitride based semipolar planes.

After finishing the growth, the semipolar GaN of block 114 is processedinto an LED. First, 2000 Angstrom (Å) to 2500 Å thick Indium Tin Oxide(ITO) is deposited, on the p-GaN of block 110, by an electron-beamevaporator for a p-type contact. ITO has higher transparency, of morethan 90% for wavelengths of light between 350 nm to 800 nm, incomparison with conventional metal p-type contacts (such asNickel(Ni)/Gold(Au), or Ni/Au, which has transparency of about 40% inthe same wavelength range). Higher light extraction is expected with theITO contact.

Once the ITO is deposited, a mesa is formed in the film of block 114 bydry-etching techniques, using Cl₂ gas for 1 to 3 minutes to expose then-type GaN. Then, the ITO layer is annealed under N₂ and O₂ ambient, ata temperature of 500° C. to 700° C. for 5 to 10 minutes, to make the ITObecome transparent. In order to form an ohmic contact to the n-type GaNlayer (n-contact), metal contacts with Titanium (Ti), Aluminum (Al), Ni,and Au layers are deposited by e-beam evaporator on the n-GaN layer.N-contacts are also annealed under N₂ ambient, at a temperature of 300°C.-500° C. for 3-5 minutes. The final step of LED fabrication is todeposit a 3000 Å to 6000 Å thick Au layer by e-beam evaporation, on then-contact and ITO for packaging purposes.

Advantages and Improvements

The existing practice is to grow GaN with the c-plane parallel to thesurface. There are several salient points that are advantageous in thepresent invention in comparison with current c-plane nitride technology.FIG. 2 shows the temperature profile (temperature vs. time) for anembodiment of the present invention, showing growth of silicon doped GaN(GaN:Si) at 1000° C., followed by growth of a SQW structure at 800° C.(first GaN barrier layer, InGaN layer, and second GaN barrier layer),followed by growth of an AlGaN electron blocking layer at 800° C.,followed by growth of Mg doped GaN (GaN:Mg) at 805° C. and 850° C.

Normally, n-GaN for a c-plane GaN film is typically deposited/grown at atemperature of 1050° C. FIG. 2, however, shows that the presentinvention deposits/grows n-type III-nitride (e.g., the n-GaN of block104) at a low temperature, 10-50° C. lower than the temperature used forgrowth of c-plane III-nitride (e.g., GaN); therefore, low temperaturegrown n-GaN is employed as well as low temperature grown p-GaN. FIG. 2also shows that the p-type III-nitride layer (e.g., p-GaN of block 110),on the MQWs, is grown at a temperature up to 250° C. lower than thetemperature used for growth of c-plane III-nitride or GaN.

Current c-plane blue LEDs are grown using 3.9-8.6 μmol/minute of TMIn,yielding typical QW thicknesses of 2-2.5 nm. During the SQW depositionin the present invention (block 106), a higher flow rate of TMIn, 10.6μmol/minute, and lower temperature (10-40° C. lower) are used, and athicker QW is grown. Consequently, the In containing QWs or MQWs ofblock 106 are grown on the n-type layer to a thickness thicker than athickness of c-plane nitride based light-emitting QW layer, at a fastergrowth rate than a growth rate used for growing the c-plane nitridebased light-emitting QW layer, and/or a lower temperature (10-40° C.lower) than a temperature used to grow the c-plane nitride basedlight-emitting QW structure. The result is a higher InN composition andhigher quality active region which allows longer wavelength emission.

The existing practice has not been able to produce yellow LEDs. FIG. 3illustrates currently available LEDs' external quantum efficiency (EQE)with respect to peak wavelength emitted, for c-plane UCSB LEDs, nonpolarUCSB LEDs, c-plane InGaN LEDs, (Al_(x)Ga_(1-x))_(0.52)In_(0.48)P LEDs,and IWN06 by Nichia. Table 1 summarizes commercially availableAlInGaP-based LEDs in the amber region. As can be seen from both FIG. 3and Table 1, there have been no yellow LEDs emitting at peak wavelengthsof 560 nm-580 nm, in either III-phosphide or In-nitride materialsystems. Red and amber LEDs have been produced by AlInGaP materialsystems for a while, and LEDs emitting light in the ultraviolet, blue,and green regions of the visible spectrum have been produced by (Al, In,Ga)N alloys (c-plane UCSB, nonpolar UCSB, and c-plane InGaN). However,prior to the present invention, no LEDs have ever been made in theyellow regions between 560 nm-580 nm wavelength in any material system.

For the first time, the present invention has achieved nitride LEDsemitting light at longer wavelength with output powers that arecomparable to longer wavelength light emission from AlInGaP-basedcommercial LEDs (the present invention's LEDs, emitting 5.9 mW at 564 nmwavelength, and 3.5 mW at 575.7 nm wavelength, are indicated by“Semipolar-UCSB” in FIG. 3). For the LED emitting light of wavelength564 nm, and the LED emitting light of wavelength 575.7 nm, the externalquantum efficiencies are 13.4% and 8.2%, respectively [6]. As shown inFIG. 3, blue LEDs emitting light at 450 nm wavelength, and green LEDsemitting light at 520 nm wavelength (LEDs on semipolar planes), usingeither {11-22} and/or {10-11} planes, have been developed by UCSB.

TABLE 1 Performance of commercially available AlInGaP based LEDs atlonger emission wavelengths, wherein TO-46 and 5s-PKG indicate packagingtechniques that were employed. Peak wavelength TO-46 5s-PKG 595 nm 2.8mW 4.3 mW 625 nm 6.5 mW >10 mW 635 nm >7 mW >10 mW 660 nm >7 mW >10 mW

FIG. 4( a) shows the dependence of LEDs' output power and peakwavelength of emission on the number of QWs. The output power of thesemipolar LEDs was evaluated by measuring the light output using asilicon photo detector through the back of the substrate. It isimportant to note that there is only a small shift in peak wavelengthsand the output power change is within 0.1 mW indicating the high qualityof QWs embodied by the growth conditions discussed earlier. Further,this result indicates more efficient growth, with less growth time, andthus InGaN's thermal budget can be greatly decreased. The presentinvention believes this is one of the reasons for successful fabricationof yellow LEDs.

FIG. 4( b) describes the results of a set of experiments to observe therelationship between thickness of QWs and the output power and peakwavelengths emitted from the semipolar LEDs. Peak emission wavelengthincreases as thickness of the QW increases, as the indium composition inthe active layer becomes higher. However, the output power, which isdirectly related to the efficiency of carrier recombination, shows anoptimal thickness of 3 nm for this particular sample, and it dropsquickly. This behavior is most likely due to poor crystal quality of theactive layer with higher In composition. By combination of high qualityQWs and an optimized QW thickness of 3 nm, the present invention wasable to produce high power LEDs emitting light having a peak wavelengthin the range 560 nm to 580 nm.

FIGS. 4( a), 4(b), 5(a) and 5(b) illustrate at least one yellow, amberor red LED, grown on at least one III-nitride-based semipolar plane,with a peak emission wavelength longer than 560 nm and an output powerof more than 3.5 mW at a current of 20 mA. FIGS. 4( a), 4(b), 5(a), and5(b) illustrate that the LEDs typically comprise an active layer foremitting light, wherein the semipolar plane enables a thickness of theactive layer that is sufficiently thick, an In composition of the activelayer that is sufficiently high, and a crystal quality of the activelayer that is sufficiently high, so that the light has a peak emissionwavelength longer than 560 nm and an output power of more than 3.5 mW ata current of 20 mA. In one embodiment, the LED has a 600×450 μm² mesasize, and the current density at 20 mA current is 20 mA/(600×450μm²)=7.4 Amps per centimeter square.

LED Structures

FIG. 6 also illustrates at least one yellow, amber or red LED 600, grownon at least one III-nitride-based semipolar plane 602, with a peakemission wavelength longer than 560 nm and an output power of more than3.5 mW at a current of 20 mA. The semipolar plane 602 is a top surface604 (or surface upon which subsequent layers are grown) of a GaN orIII-nitride substrate 606 having a semi-polar orientation (e.g., grownin a semi-polar direction 608). The III-nitride based semipolar plane602 may be a {11-22} or a {10-1-1} semipolar plane, for example,however, other semipolar planes are also possible.

The LED 600 comprises an active layer, and the active layer of the LED600 is comprised of an In containing SQW structure 610 (comprising a QW612, e.g., InGaN QW 612 between a GaN barrier layer 614 and a GaNbarrier layer 616) or a MQW structure 618 (e.g., comprising a two periodMQW structure 618 comprising the QW 612 and QW 620, e.g., InGaN QW 612between GaN barrier layers 614 and 616 and InGaN QW layer 620 betweenGaN barrier layers 616 and 622, however the MQW structure 618 maycomprise more than two periods so that the MQW structures comprise twoor more QWs 612, 620). The QWs 612, 620 are for emitting light havingthe peak emission wavelength and the output power. The peak emissionwavelength increases as an amount of In in the QWs 612, 620 increases;increasing a thickness 624 and 626 of the QWs 612 and 620 increases anamount of indium in the QWs 612 and 620. Therefore, the QWs 612 and 620have the thickness 624 and 626 sufficiently thick, to obtain the amountof In sufficiently high to emit the light having the peak emissionwavelength longer than 560 nm (e.g., amber, yellow or amber light). TheQWs typically have the thickness 624 and 626 of 2 nm to 7 nm (i.e.thickness 624 and 626 are each 2 nm-7 nm thick). The QWs 612, 620 arethicker, and therefore can emit light having a longer wavelength, ascompared to c-plane (Al,In,Ga,B)N based QWs. In one example, a numberand/or thickness 624, 626 of QWs 612, 620 in the LED 600 can be selectedto optimize (i.e. maximize) wavelength emission and output power, bychoosing the thickness 624, 626 and number of QWs 612,620 correspondingto the intersection of the output power and EL peak wavelength curves inFIGS. 4( a) and 4(b).

The QW structure 610, or MQW structure 618, is between a p-type nitride(e.g., GaN) layer 628 and an n-type nitride (e.g., GaN) layer 630. Thep-GaN 628 and n-GaN 630 provide holes and electrons, respectively, tothe QWs 612, 620 so that the electrons and holes can re-combine in theQW 612 to emit light having a minimum photon energy corresponding to thebandgap energy of the QW 612. The amount of In incorporated in the QW612 determines the bandgap of the QW, which in turn determines theminimum energy of the photons, and hence wavelength of the photons,emitted.

The LED 600 typically further comprises a p-type contact layer (e.g.,ITO) 632 making ohmic contact to the p-type GaN layer 628 and a n-typecontact layer (e.g., Ti/Al/Ni/Au) 634 making ohmic contact to the n-typeGaN layer 630. An external power supply can supply current between, orvoltage across, the p-contact layer 632 and n-contact layer 634, thussupplying the power necessary to drive the electrons and holes into theQW 612 such that the electrons and holes can re-combine to emit light.Metallization layers (e.g., Au) 636 and 638 may be on the p-contactlayer 632 and n-contact layer 634 in order to improveelectrical/mechanical contact between the LED 600 and the external powersupply.

The LED 600 may also comprise an electron blocking layer 640 (e.g.,AlGaN) between the barrier layer 622 and the p-type GaN layer 628.

Typically, layers 630, 610, 618, 640, and 628 are grown epitaxially ontop of one another, along the direction 608, and on the substrate 606(e.g., layer 630 on layer 606, layer 614 on layer 630, layer 612 onlayer 614, layer 616 on layer 612, etc.). In such a way, QW layers 612and 620 have a semipolar orientation, or a top surface 642 and 644,respectively, which has the semipolar orientation of the semipolar plane602. Typically, n-type layer 630, barrier layer 614, barrier layer 616,barrier layer 622, blocking layer 640, and p-type layer 628, have asemi-polar orientation, or a top surface 646, 648, 650, 652, 654, and656, respectively, which have the semipolar orientation of thesemi-polar plane 602.

Successful fabrication of a yellow LED has broadened the spectrum ofsemipolar based LEDs. The chromaticity diagram in FIG. 7 shows thepossibility of multi-color LEDs fabricated on a semipolar GaN substrate.Current white LEDs are made using a YAG phosphor with 450 nm (blue) LEDsshining on the YAG phosphor. Even though this is the most commonly usedtechnique for white LED fabrication, the biggest drawback of thistechnique is the 50% conversion loss in the phosphor film. The othermethod for producing white LEDs is to combine different colors of LEDs.As shown by FIG. 7, combining blue and yellow LEDs, it will be possibleto fabricate high power and high efficiency white LEDs solely based onnitride technology. In addition, high power multi-color LEDs are nowpossible by combining blue, green, and yellow semipolar based NitrideLEDs, and red AlInGaP LEDs.

FIG. 7 shows that the invention of yellow LEDs now allows the creationof multi-color LEDs by combinations of blue, green, and yellow LEDs, andhigh power white LEDs can be realized by combining blue and yellow LEDs.As shown in FIG. 3, blue LEDs emitting light at 450 nm wavelength, andgreen LEDs emitting light at 520 nm wavelength (LEDs on semipolarplanes), using either {11-22} and/or {10-11} planes, have been developedby UCSB [3,5]. The area within the triangle 700 in FIG. 7 shows thepossible colors that can be generated by combining UCSB semipolar blue(e.g., emitting light of wavelength λ=433.5 nm), green (e.g., emittinglight of λ=519.2 nm), and yellow (e.g., emitting light of wavelengthλ=564 nm) LEDs. Thus, the multi-color LED device, emitting multiplecolors of light, is comprised of the LED 600 (e.g., at least oneIII-nitride based semipolar yellow LED 600, grown on a III-nitride basedsemipolar plane 602, for example, a {11-22} plane, and emitting yellowlight), and other nitride based (or non-nitride based) LEDs emittingblue, green, red, and/or other colored, light. Similarly, a white LEDdevice, emitting white light, may be comprised of the LED 600 (e.g., atleast one III-nitride based semipolar yellow LED 600, grown on aIII-nitride based semipolar plane, for example, a {11-22} plane andemitting yellow light), and other nitride and non-nitride based LEDsemitting blue, green, red, and/or other colored light, so that thecombination of the light emitted by the LEDs is white light. Forexample, the white light emitting LED device may comprise a blue lightemitting LED and a yellow light emitting LED.

Growth of high-quality QWs emitting in a broad range of the visiblespectrum by one materials system, (Al, In, Ga, B)N, suggests anotherpossible technology. Growth of InGaN active layers of different bandgapwithin a device can be employed to create a single white-LED structure.The band structure of such a white LED structure 800 is shown in FIG. 8,which illustrates the conduction band E_(c), valence band E_(v), MQWstructure 802 between a semipolar n-type GaN (n-GaN) layer 804 andsemipolar p-type GaN (p-GaN) layer 806, wherein the MQW structure 802comprises a QW 808 (with an In composition x, e.g., In_(x)Ga_(1-x)N) foremitting blue light 810, and QWs 812, 814 (with an In composition x,e.g., In_(x)Ga_(1-x)N) for emitting yellow light 816, 818 and barrierlayers 820, 822 (e.g. GaN). The band structure of QWs 808 is carefullydesigned so that dissociation of In in the blue-active-region 808 isminimal and there is higher hole injection for the yellow-active-region812, 814. A SQW can be employed for the blue-active-region 808, and MQWsfor yellow-active-regions 812, 814 to increase the efficiency of yellowemission 816, 818. With a flip-chip packaging technique and this bandstructure, a white LED can be realized on a single device. The bluelight 810 is emitted with an energy equal to at least the bandgap energy(E_(c)-E_(v)) in the QW 808, and the yellow light 816, 818 is emittedwith an energy equal to at least the bandgap energy (E_(c)-E_(v)) in theQWs 812, 814.

FIG. 9, which is based on FIG. 6, illustrates a white LED 900 comprisedof one or more blue light emitting QWs 902 and one or more yellow lightemitting QWs 612, 620 grown on one or more III-nitride based semipolarplanes 602, in a single growth structure, for example. As such, the LED900 comprises MQWs that are comprised of the blue-light emitting QW 902and the yellow-light emitting QW 612. Typically, the blue-active-region904 of the LED 900 comprises at least one blue-light emitting QW 902 foremitting blue light (e.g., a first InGaN QW with a first In compositionand thickness 908 for emitting blue light), between a first GaN barrierlayer 906 and a second GaN barrier layer 614; the yellow-active-region610 comprises a yellow-light emitting QW 612 for emitting yellow light(e.g., a second InGaN QW with a second In composition and thickness 624,626 for emitting yellow light), between the second GaN barrier layer 614and a third GaN barrier layer 616; and the one or more III-nitridesemipolar planes 602 are a surface 646, 604 of a III-nitride n-typelayer 630 or substrate 606 (however, the semipolar planes 602 may besurfaces of other layers). The blue-active-region 904, andyellow-active-region 610, 618 are typically between the n-type layer 630and p-type layer 628. As noted above, the yellow-active-region 618 isusually (but not limited to) a MQW structure and the layers 610, 904,618, 640, 628, and 630 in FIG. 9 are typically grown epitaxially one ontop of the other and have a semipolar orientation 608.

In the embodiments of FIG. 6 and FIG. 9, the In content, and thickness624, 626, 908 of QWs 612, 620, and 902, determine the minimum bandgapof, and hence wavelength of light emitted, by the QWs 612, 620, and 902:in those examples, the In content and thickness 908 is selected suchthat active layer 902 emits blue light (or a wavelength corresponding toblue light) and the In content and thicknesses 624, 626 are selectedsuch that active layers 612, 620 emit yellow, amber and/or red light (ora wavelength corresponding to yellow, amber, and/or red light). However,the blue-active-region 904 may be replaced with an active regionemitting other colors of light, or additional active regions emittingother colors of light may be introduced/grown in between the n-typelayer 630 and p-type layer 628, in order to obtain an LED deviceemitting multi-colors (multi-color LED device) or white light (white LEDdevice). The active region emitting other colors of light requires QWshaving an In incorporation x (in e.g., In_(x)Ga_(1-x)N) and thicknessthat allows the QW to emit the other colors of light.

FIG. 10 shows a representative cross-sectional TEM image of a semipolarInGaN QW structure on a (11-22) GaN substrate, showing n-type GaN:Silayer 1000, InGaN QW layer 1002 on the GaN:Si 1000, GaN cap layer 1004on the InGaN QW 1002, AlGaN electron blocking layer 1006 on the GaN caplayer 1004, p-type GaN:Mg layer 1008 on the AlGaN electron blockinglayer 1006, low TD densities (<5×10⁶ cm⁻²) and a planar InGaN QW 1002with abrupt interface 1010, 1012. Increasing a crystal quality of theQWs 612 and 620 increases the output power of the device 600; forexample, the QWs 612 and 620 have the crystal quality sufficiently highto obtain the output power of more than 3.5 mW at a current of 20 mA, ormore than 29.2 mW at 200 mA. FIG. 10 illustrates that the crystalquality is, or can be measured by, TDs, a planar QW and/or an abruptinterface 1012. Thus, output powers of more than 3.5 mW at a current of20 mA can be obtained with TD densities 5×10⁶ cm⁻² or less, or even9×10⁹ cm⁻² or less.

REFERENCES

The following references are incorporated by reference herein.

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CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

1. A semiconductor Light Emitting Diode (LED), comprising at least: an-type layer; a p-type layer; an active layer between the n-type layerand the p-type layer, wherein: the active layer emits light with a peakemission wavelength longer than 560 nm at a current density of at leastless than 7.4 Amps per centimeter square, and the LED is a III-nitridebased semi-polar LED.
 2. The LED of claim 1, wherein the active layerfurther includes Indium and an emission color of the LED corresponds toyellow light.
 3. The LED of claim 2, wherein: the LED is grown on asemipolar plane of a bulk GaN substrate, and the active layer issufficiently thick, an Indium (In) composition of the active layer issufficiently high, and a crystal quality of the active layer issufficiently high, to emit the light with an output power of more than3.5 mW at the current density of 7.4 Amps per centimeter square and withan External Quantum Efficiency of at least 8.2%.
 4. The LED of claim 2,wherein the active layer comprises a threading dislocation density ofless than 5×10⁶ cm⁻².
 5. The LED of claim 2, wherein the active layercomprises a dislocation density of less than 5×10⁶ cm⁻².
 6. The LED ofclaim 1, wherein the active layer includes one or more planar quantumwells comprising abrupt interfaces.
 7. The LED of claim 6, wherein thequantum wells have a thickness in a range from 2 nm to 7 nm.
 8. The LEDof claim 1, wherein the light is emitted with an output power of morethan 3.5 mW at the current density.
 9. The LED of claim 8, wherein thelight is emitted with an External Quantum Efficiency of at least 8.2%.10. A multi-color LED device comprised of the LED of claim
 1. 11. Awhite LED device comprised of the multi-color LED device of claim 10,wherein the white LED device emits white light.
 12. The multi-color LEDdevice of claim 10, wherein the white light is emitted without aphosphor.
 13. The multi-color LED device of claim 10, further comprisinga blue LED combined with the LED of claim
 1. 14. The multi-color LEDdevice of claim 10, further comprising at least an additional activelayer between the n-type layer and the p-type layer, wherein theadditional active layer emits additional light that, in combination withthe light, forms white light.
 15. The LED of claim 1, further comprisinga transparent Indium Tin Oxide layer on the p-type layer.
 16. A methodof fabricating a semiconductor Light Emitting Diode (LED), comprising atleast: depositing an n-type layer; depositing a p-type layer; anddepositing an active layer between the n-type layer and the p-typelayer, wherein: the active layer emits light with a peak emissionwavelength longer than 560 nm at a current density of at least less than7.4 Amps per centimeter square, and the LED is a III-nitride basedsemi-polar LED.
 17. The method of claim 16, wherein the depositing is byMetal Organic Chemical Vapor Deposition (MOCVD) and: the n-type layercomprises an n-type Gallium Nitride (GaN) layer deposited on asemi-polar plane of a GaN substrate, the n-type GaN is grown at atemperature 10-50° C. lower than a temperature used for growth ofc-plane GaN, the active layer: (1) comprises an Indium containingquantum well grown at a temperature lower than the temperature at whichthe n-type layer is grown, (2) the quantum well is grown to a thicknessthicker than a thickness of c-plane nitride based light-emitting quantumwell layer, (3) the quantum well is grown at a faster growth rate than agrowth rate used for growing the c-plane nitride based light-emitting QWlayer, the p-type layer comprises a p-type GaN layer grown on a p-typeelectron blocking layer, wherein the p-type electron blocking layer isgrown on the active layer, the p-type GaN layer is grown at atemperature up to 250° C. lower than a temperature used for growth ofc-plane III-nitride or GaN.
 18. The method of claim 17, furthercomprising: depositing Indium Tin Oxide on the p-GaN; and annealing theITO to make the ITO become transparent.
 19. The method of claim 16,wherein: the LED is a III-nitride based semi-polar LED, the LED is grownon a bulk GaN substrate, the active layer further includes Indium, andthe depositing is under conditions such that the active layer issufficiently thick, an Indium (In) composition of the active layer issufficiently high, and a crystal quality of the active layer issufficiently high to emit the light with an output power of more than3.5 mW at the current density and with an External Quantum Efficiency ofat least 8.2%.
 20. The method of claim 16, wherein the depositing issuch that the active layer includes one or more planar quantum wellscomprising abrupt interfaces.
 21. The method of claim 16, wherein theactive layer further includes Indium and an emission color of the LEDcorresponds to yellow light.